1. Field of the Invention
The invention relates to an electric power conversion system that is able to step up or step down electric power in parallel with two direct-current power supplies.
2. Description of Related Art
In a hybrid vehicle or an electric vehicle, which uses a rotary electric machine as a driving source, the rotary electric machine is driven by alternating-current power that is converted by an inverter from the direct-current power of a battery. In addition, a step-up/step-down converter is provided between the battery and the inverter. The step-up/step-down converter steps up a battery voltage or steps down electric power regenerated by the rotary electric machine.
An electric power converter is, for example, described in Japanese Patent Application Publication No. 2013-013234 (JP 2013-013234 A) as the one that extends the function of the step-up/step-down converter. The electric power converter includes four switching elements, and is connected to two batteries. The electric power converter is able to switch the two batteries between series connection and parallel connection in addition to the step-up/step-down function.
The above-described electric power converter steps up or steps down electric power in parallel with the two batteries at the time of the parallel connection (parallel mode). Step-up/step-down operation is controlled via a PWM signal indicating a duty ratio to each of step-up/step-down circuits. The electric power converter described in JP 2013-013234 A has such a circuit configuration that switching elements are shared between the two step-up/step-down circuits, so each of the switching elements operates in accordance with the logical addition of PWM signals of both.
By utilizing the above operation characteristics of the switching elements, the phases of the PWM signals are shifted from each other in JP 2013-013234 A to reduce a loss that arises in the switching elements. That is, as shown in FIG. 17C, an edge-alignment phase shift is carried out. The edge-alignment phase shift is to shift one or both of the phases of the PWM signals PWM1, PWM2 such that the falling edge of an on period of the PWM signal PWM1 is brought into coincidence (connected) with the rising edge of an on period of the PWM signal PWM2. Thus, the number of times of switching (indicated by the broken line) is reduced as compared to the PWM signal shown in FIG. 17A, with the result that a switching loss is reduced. Because the overlap (hatched portion) of the on periods of the PWM signals PWM1, PWM2 as shown in FIG. 17B disappears, a steady loss (overlap loss) due to an increase in current resulting from the overlap is resolved.
When the edge-alignment phase shift is carried out, as shown at the top of FIG. 18, only merely shifting the rising timing of a carrier signal results in that, as indicated by the hatching in PWM2-2, the on period of PWM2 extends into the third control period, so the on period of PWM2 in the second period becomes shorter than a required value. In order to compensate for such a shortened on period within a phase-shift period, an auxiliary carrier having a period corresponding to the amount of phase shift as shown at the bottom of FIG. 18 is inserted in the existing art. By inserting the auxiliary carrier, an on period provided by an auxiliary pulse arises in PWM2 within the second period, with the result that the above-described shortened on period is compensated.
Incidentally, in a period after the edge-alignment phase shift, if the on period of the PWM signal becomes short as in the case of the PWM signals PWM1-3, PWM2-3 shown in FIG. 19, the rising edge of the on period of the PWM signal PWM2 separates (delays) from the falling edge of the on period of the PWM signal PWM1 as in the case of the PWM signals PWM1-3, PWM2-3 shown in FIG. 19.
When the phase shift is executed again as shown at Auxiliary 2 in FIG. 20, an auxiliary pulse based on the auxiliary carrier arises in the PWM signal as surrounded by the broken lines in FIG. 20, so a switching loss due to the auxiliary pulse arises. That is, as a result of execution of the phase shift, a switching loss due to the auxiliary pulse arises.
In the case where the on period of the PWM signal changes over multiple periods, for example, in the case where the on period of the PWM signal is gradually reduced, if the phase shift is executed by inserting an auxiliary carrier each time the on period is reduced, a switching loss due to an auxiliary pulse frequently arises. As a result, a reduction in loss by connecting the on periods of the PWM signals PWM1, PWM2 is cancelled out by an increase in loss due to an auxiliary pulse, and there is a concern that the effect of a reduction in loss that is the original purpose of the phase shift is not sufficiently obtained.